Carbon export in an Arctic frontal system in Fram Strait

Recent studies report high carbon export efficiencies in Arctic regions with seasonal sea ice compared to ice-free regions, yet the mechanisms behind this enhanced export remain unclear. In the marginal ice zone (MIZ), where pack ice meets open ocean, eddies and filaments frequently form. To investigate carbon export mechanisms in such frontal systems, we combined direct in situ observations of export processes and physical oceanography using free-drifting sediment traps, Marine Snow Catchers, and in situ optics during a Fram Strait expedition in July 2020. Dense Atlantic Water (AW) subducted beneath lighter surface meltwater and Polar Water (PW), structuring biogeochemical and biological processes, including chlorophyll distribution, primary production, zooplankton abundance, microbial respiration, and aggregate formation. We observed three main mechanisms supporting carbon export; (i) deep aggregate formation driven by the subduction of chlorophyll-rich AW, (ii) diatom ballasting of aggregates, including slow-sinking Phaeocystis colonies, and (iii) cryogenic mineral ballasting in PW aggregates, which increased sinking velocities up to tenfold. High carbon fluxes occurred in the AW and at the front, while export in PW was lower, highlighting the role of water mass characteristics and ballasting in regulating export efficiency. The results suggest that continued Atlantification and expansion of the MIZ could enhance carbon export across larger Arctic areas as sea ice retreats.